The present invention relates to wheelchairs, particularly to wheelchair frames for supporting the wheels, seat and foot rest assemblies.
Hand-propelled wheelchairs generally include a frame supporting a pair of drive wheels at the rear end and a pair of caster wheels at the front end. The drive wheels and the caster wheels are generally rigidly supported by the wheelchair frame. The drive wheels make contact with the ground and are hand-driven to propel the wheelchair. The wheelchair supports a seat assembly comprising a seat and a backrest. The seat assembly is oriented above and between the rear drive wheels and the front caster wheels so as to provide stability.
Motor-driven wheelchairs differ in that the front and rear wheels are typically of the same or similar size. The front wheels are controlled by a steering means operable by the user, rather than being caster wheels. Further, in order to support the motor and batteries, the wheelchair frame is typically built more rugged.
Many wheelchairs utilize rigid frames. Indeed, rigid frames present the advantage of being light weight and simple. However, problems arise when a wheelchair utilizing a rigid frame is required to negotiate obstacles or uneven ground surfaces; the drive wheels and the caster wheels may fail to maintain full contact with the ground. A rigid framed wheelchair may be more difficult to propel over obstacles. Further, the relative height of wheelchairs can result in precarious angles of lean as the wheels roll over obstacles, in the absence of means for independent vertical travel of the wheelchair. Moreover, the rigid frame tends to torque in an effort to conform to uneven surfaces. These torsional stresses on the main frame take their toll over many minor cycles and result in frame failures. Various attempts have been made to overcome this problem, specifically, various devices have been proposed for minimizing the forces that are transmitted through the chair to the user.
One attempt consists of modifying an existing wheelchair in order to incorporate a shock absorbing device into the initial design of the wheelchair. For example, U.S. Pat. No. 4,572,533 to Ellis et al. discloses a shock absorbing apparatus comprising one or more spring assemblies attached to the frame in order to cushion shock on respective wheels of the wheelchair. U.S. Pat. No. 5,855,387 to Gill et al. discloses a wheelchair with multiple shock absorbing means. A rear swing axle is pivotally connected to the seat frame and a spring shock absorber controls the relative movement therebetween. A front suspension is connected to the seat frame separately from the rear suspension and includes a floating beam carrying the front castor wheels and pivotally connected to the seat frame by four link control arms. Three shock absorbers acting between the front of the seat frame and the floating beam allow controlled vertical movement of the floating beam. Numerous other patents disclose wheelchairs equipped with shock absorbing devices including U.S. Pat. No. 4,190,263 to Powers; U.S. Pat. No. 4,078,817 to Ferguson et al.; U.S. Pat. No. 4,861,056 to Duffy, Jr. et al.;U.S. Pat. No. 4,455,031 to Hosaka; and U.S. Pat. No. 3,917,312 to Rodaway. However, it has been found that such shock absorbing devices add weight and become maintenance problems.
Another attempt to provide a wheelchair with good contact between the wheels and the ground surface is described in U.S. Pat. No. 4,128,137 to Booth. This patent discloses a suspension comprising a plate carrying a wheel unit and a bogie system. The bogie system comprises two bogie units. Each bogie unit includes a drive wheel and a caster wheel attached to a frame member. The frame members pivot about a common transverse axis defined by brackets mounted to the plate to maintain contact with the ground surface.
In another attempt to ensure that a wheelchair retains good stability, a wheelchair with a frame having the capacity to flex has been envisaged. U.S. Pat. No. 5,064,211 to Huttenuis et al. discloses a flexible wheelchair frame comprising two side frame members, joined together by front and rear frame members which freely rotate relative to the side frames to permit flexure of the frame.
A central member joining the front and rear members causes the side frames to pivot in such a way that tilting of one of the side frames in one direction causes tilting of the other side frame through the same angle in the other direction, while the central member remains stationary.
One drawback to the arrangement in Huttenuis et al. is that in the absence of a monolithic rear cross member with a large box like sectional configuration it is difficult for the wheelchair to bear the weight of a heavy passenger. In order to compensate, other frame components must be enlarged, rendering the overall structure heavy and complex to manufacture.
A need that is not well addressed by the prior art, is to provide a wheelchair frame that freely torques or flexes to accommodate irregularities of the ground while maintaining wheelchair stability, while also being relatively simple and inexpensive to manufacture. A further need is to provide a convenient means to adjust the wheelbase of a wheelchair. This need is not addressed in the art.
The present invention is a wheelchair frame that has the capacity to flex sufficiently to absorb ground irregularities, while alleviating at least some of the drawbacks of the prior art. The wheelchair frame comprising side rails as well as front and rear cross members. The side rails are able to rotate relative to the front and rear cross rails in such a way as to permit the front and rear cross members to independently tilt go out of parallel alignment when viewed from the front or back.
One embodiment of the invention is a wheelchair frame assembly having a front end and a rear end and lateral sides, said wheelchair frame assembly comprising:
a first of the cross members comprising a rigid member having an aperture on either end thereof to receive the side rails such that both of the rails may rotate within the apertures and the cross member may adjustably move along the side rails to change the effective wheelbase length of the frame; and
a second of the cross members being rotatable axially relative to said side rails to permit the side rails to angle vertically independently of each other.
The first cross member thus rotates about the axis of each side rail, while the second of cross member rotates about its own axis relative to the side rails.
Preferably, the first cross member is at the rear of the frame and the second cross member at the front.
The second cross member preferably comprises a tube-in-tube arrangement rigidly joined at either end to the side rails. Preferably, an elastic sleeve such as a plastic tube joins the two components of the second cross member to provide anti-torque bias to the member.
When assembled into a wheelchair, caster wheels are mounted to the front end of the frame and drive wheels are mounted to the rear.
The invention further comprises a wheelchair having a frame as defined above, along with a seat assembly, wheels and optionally a footrest assembly.